Platinum carbon rods for electron microscope technique



United States Patent PLATINUM CARBON RODS FOR ELECTRON MICROSCOPE TECHNIQUE David Edward Bradley, Tilehurst, Reading, England, as-

signor to Metropolitan-Vickers Electrical Company Limited, London, England, a company of Great Britain No Drawing. Filed Aug. 17, 1959, Ser. No. 833,990

Claims priority, application Great Britain Aug. 20, 1958 2 'Claims. ('Cl. 252-503) This invention is concerned with the methods employed to facilitate the study of carbon replicas of specimens under the electron microscope, and is particularly concerned with the preparation of platinum/carbon rods which can be employed to produce a deposit of carbon and platinum on the surface of a specimen.

When a carbon replica of the surface of a specimen is made for examination in the electron microscope, it is necessary to enhance the contrasts of the replica by means of the technique known as shadowcasting. Conventional shadowcasting consists of evaporating a heavy metal from a filament onto the specimen at an angle, so casting shadows of the surface irregularities. The disadvantage of metals is that they form minute aggregates, so limiting the size of the fine detail which can be seen.

It has been found that the simultaneous evaporation of platinum and carbon provides a deposit which does not form aggregates, and yet has a high enough mean atomic number to give good shadows when the deposit is 20-30 angstrom units in thickness.

The simultaneous evaporation of platinum and carbon is achieved by passing a heavy alternating current (about 50 amps.) through two electrodes in the form of pointed rods, with the points lightly sprung together. The rods consist of a bonded mixture of platinum and carbon. The whole assembly is placed in a vacuum plant. The current causes local heating at the points, and the carbon and heavy metal are vaporized.

The present invention consists in a method for bonding platinum and carbon powders into hard rods, which comprises coating all the particles in a mixture of the metal and carbon powders with organic compounds, extruding the mixture after compression using oil as a binder, and finally baking the rods to remove oil and decompose the organic compounds. The method is based on the formation of interlacing graphite particles or crystals produced by the breakdown of organic materials. The proportion of platinum to carbon may range from 3:1 to 5:1 by weight.

The invention will be readily understood from the following detailed example of the method. Platinum is used here, but the method is not limited to any one metal.

Example (a) Finely divided platinum powder and carbon powder are well mixed in a pestle and mortar (convenient weights, 2.0 gm. platinum, 0.6 gm. carbon).

(b) A aqueous solution of acacia (gum arabic) is now added, 0.45 ml. per gm. of mixture (1.0 ml. for the above weights of platinum and carbon). The acacia provides the bulk of the organic material for binding purposes. The powders and solution are mixed to a paste; a drop of acetone may be added to assist in mixing it the powder is not wetted. The paste is now parially dried with a stream of hot air.

(0) A similar quantity of a solution of pitch (or W wax) in chloroform is added together with 1 drop of linseed oil (80 poises) per gm. of original platinum/car- 2,985,599 Patented May 23, 1961 bon mixture (with the above quantities, two drops are required). The pitch helps to hold the rods together before baking, and the linseed oil facilitates the extrusion described below. A few ml. of ether added at this stage assists in mixing the components thoroughly. This paste is dried as much as possible with hot air.

(d) The mixture, now in the form of lumps of material resembling plasticine in texture, is broken down as much as possible. It is then placed in a test tube and warmed gently over a Bunsen, shaking it until all excess moisture and solvents are driven off. As soon as any smoke appears, the test tube is removed from the heat. The powder is now returned to the pestle and mortar and reground. After this stage it is still rather coarse.

(e) The powder is next fed into a simple press, which consists of a short length of /2 stegl rod with a %2" hole drilled down the whole length. About A" of silver steel rod is inserted in one end to act as a bung, and the powder fed into the other end. When the tube is full of powder a drop of 10% linseed oil in benzene is introduced to prevent the material from sticking to the walls of the tube. A longer piece of silver steel rod is now inserted into the tube, and the whole placed endwise in a vise so that the mixture can be compressed. When the vise has been tightened as much as possible without bending the silver steel rod, it is loosened, and the assembly moved so that the bung can be pushed out and the mixture extruded. The resulting rod is somewhat soft and must be handled with care.

(f) The next stage consists of baking the rod, prior to curing. The soft, freshly extruded rod is allowed to dry in air for about an hour to remove as much benzene as possible. It is then inserted in a length of glass tubing through which nitrogen or any inert gas can be passed. It is heated in a stream of inert gas, gently at first, over a 450 watt electric Bunsen until all the linseed oil has been driven ofr (about 20 minutes). This baking is essentially for distilling ofi? unwanted liquids. Because of the risk of combustion, it is not desirable to initiate the decomposition of the organic compounds at this stage, hence the reason for carrying out the process in an inert atmosphere.

(g) The curing process is carried out after cooling the baked rod and removing it from the glass tube. It consists of heating the rod in a V-shaped molybdenum foil boat at a pressure of about 10- mm. of mercury. It is necessary to heat the rod to at least 500 C., and this can be achieved by heating the boat to over 1000 C. The rod can be observed to glow cherry red if the heating current is switched oif, since it cools down slowly compared to the boat. The curing should be carried out for about 20 minutes.

When the rod is cool, it is removed from the vacuum and is sharpened in a drill using grade 0 emery paper.

Platinum/carbon rods made in this way have been found to be hard, and to produce a platinum/carbon film containing a high percentage of platinum when evaporated. If the maximum amount of metal stated is exceeded, the deposit produced by evaporation granulates too easily in the electron beam, and difliculty is experienced with the evaporation. Thus, it is thought that the ratio of 5 :1 for platinum to carbon represents the maximum permissible quantity. A typical analysis of the platinum/carbon rods is 83.3% platinum, 16.7%

carbon.

I What Iclaim is:

1. A carbonaceous electrode consisting of a compressed mixture of finely divided platinum and carbon in which the proportion of platinum to carbon ranges from 3:1 to 5:1 by weight.

2. A carbonaceous electrode consisting of a com pressed mixture of finely divided platinum and carbon by Weight.

References Cited in the file of this patent UNITED STATES PATENTS Little et a1. May 11, 1915 Thrune Jan. 13, 1942 Hensel et a1. Sept. 1, 1942 Ramadanofi Sept. 7, 1948 Bailey Jan. 15, 1952 Sheer et a1. Nov.-4, 1952 Greaves 'et"=al. ".1 June 2, 1953 Gartland July 16, 1957 

1. A CARBONACEOUS ELECTRODE CONSISTING OF A COMPRESSED MIXTURE OF FINELY DIVIDED PLATINUM AND CARBON IN WHICH THE PROPORTION OF PLATINUM TO CARBON RANGES FROM 3:1 TO 5:1 BY WEIGHT. 